We present a new method which can be used to forecast the variations of solar indices on the time scale of a half solar rotation. This method uses the Lyα interplanetary glow data obtained by the SWAN instrument on SOHO. We show how the maps of solar Lyα flux distributions derived from the SWAN data can be linked to the variation of activity on the solar disk. Active regions which are known to be brighter in Lyα radiation than the quiet sun illuminate more interplanetary H atoms through resonance scattering. This excess of illumination related to active regions is clearly seen in full-sky Lyα maps recorded by the SWAN instrument on SOHO. These maps include also those excesses resulting from active regions which are on the far side of the Sun, i.e. not visible to solar disk imagers near Earth. From these data, we can derive a farside to nearside flux ratio. This ratio is then used to predict the evolution of solar indices like the solar Lyα flux seen at Earth, the 10.7cm radio flux or the MgII solar index. This technique could be used in the future to improve the quality of space weather forecast, in particular to predict atmospheric heating and increased orbital drag of sensitive spacecraft.